The young of a given species of animal, in the normal course of events, develop the behavior characteristic of their kind. Yet, both within one species and particularly between species, there are vast differences in the degree of stereotypy shown in behavior. Differences in degree of behavioral stereotypy (or plasticity) appear to be related both to the stability of the environment in which the species have evolved as well as to the stability of the immediate environment in which the individual was reared. It is our aim to specify the manner and mechanisms whereby environmental factors exert their control over the evolution and development of species-characteristic behavior. The studies described herein are continuations of those begun at the Duke University Field Station for Animal Behavior Studies in 1961. Their general aims and significance remain unchanged, though certain specific objectives, now met, have been replaced by others.